Polypropylene (PP) is a widely used thermoplastic polymer known for its excellent chemical resistance, low density, and good mechanical properties. When processing PP, it's essential to follow specific temperature settings and injection parameters to ensure optimal part quality and process stability.
The barrel temperature settings are as follows:
- Feeding zone: 30–50°C (recommended at 50°C)
- Zone 1: 160–250°C (recommended at 200°C)
- Zone 2: 200–300°C (recommended at 220°C)
- Zone 3: 220–300°C (recommended at 240°C)
- Zone 4: 220–300°C (recommended at 240°C)
- Zone 5: 220–300°C (recommended at 240°C)
- Nozzle: 220–300°C (recommended at 240°C)
The recommended melt temperature range is 220–280°C, with the barrel constant temperature set at 220°C. The mold temperature should be maintained between 20–70°C. Injection pressure typically ranges from 80–140 MPa (800–1400 bar), though for thin-walled packaging containers, it can go up to 180 MPa (1800 bar). However, high injection pressure should be avoided unless necessary, as it may cause warping or stress in the final product.
Holding pressure is usually around 30–60% of the injection pressure and should be applied for about 30% of the total cycle time to prevent sink marks and shrinkage. Back pressure is typically set between 5–20 MPa (50–200 bar) to ensure proper mixing and melting of the polymer.
For injection speed, high speeds are recommended for thin-walled packaging, while medium speeds are more suitable for general-purpose parts. The screw speed can be set high (up to 1.3 m/s), provided that the plasticizing stroke is measured between 0.5D and 4D before cooling begins. A 4D stroke allows sufficient residence time for the melt, ensuring consistent quality.
The residual material in the barrel should be kept between 2–8 mm, depending on the application. Pre-drying is generally not required if storage conditions are good, but if the material has been exposed to moisture, drying at 80°C for one hour can restore its properties fully. The shrinkage rate of PP is relatively high, ranging from 1.2% to 2.5%, and it does not continue to shrink after 24 hours post-molding.
Gate systems commonly used for PP include point gates, multi-point gates, heated hot runners, insulated hot runners, and internal sprues. The gate should be placed at the thickest section of the part to minimize shrinkage and ensure even filling. For packaging containers, special screw designs with L/D ratios of 25:1 are often used, along with straight-through nozzles and check valves to enhance flow and reduce defects.
When cleaning the barrel, it’s important to use appropriate procedures, especially when switching materials or during long shutdowns. Standard screws and three-stage screws are suitable for most PP applications, but specialized configurations may be needed for high-performance packaging products.
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